The usage of capillary columns for GC first began in the late 1950s. Although there have been significant iterative advancements in the six decades since, the fundamental capillary format has remained essentially the same. Column capacity remains limited by the small diameter within the capillary, and the inverse relationship between column efficiency and column diameter. Although column efficiency increases as the column diameter decreases, column capacity decreases exponentially with decreasing diameter, and back pressure increases exponentially with decreasing diameter. For example, decreasing the inner diameter of a capillary column from 0.32 mm to 0.25 mm results in a reduction in column capacity of about 40%. Thus, the column efficiency is effectively limited by countervailing process constraints.
Multibore capillary columns have been proposed many times in the six decades since the introduction of the capillary column for GC, however these multibore capillary columns have never been successfully commercialized due to significant drawbacks in their implementation. One disadvantage of multibore capillary columns is that each bore must have the same passage rate (dead time), and the acceptable variance cannot exceed 0.1%, which is difficult to achieve. Further, back-pressure still increases exponentially with reduction of diameter, and so each bore gives rise to significant back pressure. The necessary compromises inherent in multibore capillary columns have proven to be untenable.
Another proposal for improving column capacity while maintaining column efficiency, based on theoretical calculations, has been a capillary with a rectangular cross-section. In order to overcome differential drag at the ends of the rectangular cross-section, it has been proposed to bend the ends of the rectangular channel around to meet one another, forming an annulus, but an arrangement to support the center core of the annulus running throughout the capillary has proved elusive. Both fiber-in-capillary and helical fiber supports (see U.S. Pat. No. 4,665,163) have failed to provide useful and effective structures.
Accordingly, it would be desirable to provide capillary columns and methods for forming capillary columns not suffering from the above-described drawbacks.